A mixer is a three port network which translates an incoming signal at one frequency to some other intermediate frequency. To affect this translation, the incoming signal is heterodyned or mixed in a non-linear device usually with another signal generated by local oscillator. This process generates two primary intermediate frequency signals, having frequencies which are equal to the sum and difference of the incoming signal frequency and the local oscillator frequency. However, other unwanted frequency components and products are also generated as well as the mixing of adjacent channel frequencies which also takes place.
Single or double balanced mixers when used in the tuners of television receivers have several advantages over active mixers which often use bipolar or MOSFET transistors for the VHF band and a single Schottky diode for the UHF band. These advantages include improved channel 6 beat performance, improved cross-modulation performance, improved half-IF performance, partial cancellation of local oscillator energy at the signal input port, and sufficiently wide bandwidth for use of one mixer over a plurality of bands, such as UHF and VHF. One of the reasons for these advantages of a passive mixer over an active mixer is that for an active mixer the unwanted cross-modulation products, intermodulation products and harmonics are amplified during the modulation process and it is then too late to do much beneficial filtering.
In many television receivers the IF stages comprise a surface acoustic wave (SAW) filter which provides excellent selectivity without requiring the alignment of coils but has a large insertion loss of the order of 20 db. Accordingly, it is often the practice to provide about 20-26 db amplification prior to the SAW filters to make up for this insertion loss in order to have the level of the output signal from the SAW be no lower than the signal output level from the mixer in order to maintain a satisfactory signal to noise ratio. Since the tuner RF bandpass filter circuits have relatively wider bandwidth than a single channel, substantial adjacent channel signal is present in the output of the mixer and is fed to the SAW preamplifier.
This adjacent channel signal causes increased cross modulation and intermodulation products in the mixer which are aggrevated in an active mixer. Additionally, this adjacent channel signal when applied to the SAW preamp can overload the SAW preamp or at least adds sufficient extraneous signal level to drive the SAW preamp through a larger dynamic response characteristic and thus increase the distortion of the preamp. However, even if such a preamp is not used, it is still desirable to protect the IF stages from this extraneous signal for reducing the dynamic range of the IF stages and thus reduce the distortion products.
The adjacent channel signals present more of a problem for the VHF television channels of channels 2-13 than for the UHF channels primarily because the UHF stations broadcast with at least 6 channels of separation in any particular market area. In such a case, even though the UHF tuners use a single Schottky diode as a mixer with a following bandpass filter, such tuners are not balanced and thus do not provide the lower cross-modulation, intermodulation, and harmonic products achievable with a balanced mixer in the VHF band.
Accordingly, it is desirable to provide a passive mixer such as a single or double balanced mixer for a television receiver wherein the unwanted frequency components, harmonics of the fundamental signals, and distortion products conveyed to the IF stages are reduced.
As used herein, the term television receiver includes television processing apparatus without limitation as to the presence or absence of a video display, e.g., a television set, a VCR, etc.